Abstract

In an attempt to determine to what extent the impact of recent climate changes that occurred east of Hudson Bay, including important reduction of areas in permafrost, had on the coastal marine environment, a series of shallow cores were extracted from the seabed off the mouth of the Sheldrake River, in Nastapoka Sound. A total of 25 cores were taken in April 2009 from the ice cover. A preliminary seafloor map was first done to help in the selection of the coring sites. Nastapoka Sound has a complex subaqueous relief, formed of asymmetric ridges and deep basins. After preliminary sedimentological analyses, six of the cores were selected for physical and chronological analyses (14C, 210Pb, and 137Cs dating); among them, three were selected for elemental (C, N, OC/TN) and isotopic (δ13C, δ15N) analyses to identify sedimentary organic matter sources. Sedimentation processes are complex and are primarily driven by bottom currents. This is confirmed by the absence of a clay fraction in several cores, some erosion surfaces in a few cores, and the presence of comet-like mark features on the seafloor. The highest sedimentation rates are found near the coast of the province of Quebec and Gillies Island and the lowest ones occur in the deep depocenters. Isotopic and elemental analyses reveal that Nastapoka Sound is an area of mixing between marine and terrestrial inputs and can be compared to an estuarine system similar to nearby Lac Guillaume-Delisle. Those conditions altogether make it difficult to extract a perfectly clear signal of climate change in the recent sediment record. However, the downcore application of a simple two end-member mixing model to measured δ13C values strongly suggests that the fraction of sedimentary organic matter from terrestrial sources increased by 30% since about the middle of the Little Ice Age. This trend accelerated at the end of the 20th century. Rapid permafrost decay in adjacent river catchments is likely one source for this terrestrial carbon. However, it cannot be distinguished from other potential sources that are also related to environmental changes such as increase in primary productivity both on land, where peatlands, shrubs, and forest are expanding, and at sea, where sea ice cover duration is diminishing.

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